Sulfur has long been a popular, inexpensive and effective fungicide that grape growers use for controlling powdery mildew.
Perhaps more importantly, there has never been any documented disease resistance to sulfur.
But like any tool, it has its limitations. By itself sulfur isn’t necessarily the best choice in all situations, especially those involving the more challenging powdery mildew problems.
In many cases, it must be applied as frequently as every 7 to 10 days, and that can be application-expensive and inconvenient. Sulfur is also easily washed off by rain or overhead irrigation, and it can burn leaves and injure fruit, if applied just before or on days when the temperatures reach 100 degrees or higher.
Sulfur residues on grapes can also inhibit yeast during the fermentation process and cause off-flavors in wine. Some wineries limit sulfur applications on contracted grapes.
Sulfur can also injure vines and berries if applied with a mineral oil or within three weeks of an oil spray. It can kill beneficial insects, spiders and predator mites, too. In fact, treating vineyards with sulfur can actually lead to a buildup of harmful pests such as spider mites, leafhoppers and vine mealybugs
Then, there’s the problem of using sulfur against heavy powdery mildew pressures — it may not work in that situation.
That’s when a synthetic fungicide can prove valuable as part of a progressive disease management strategy, says Patrick Dosier, market development manager for MANA.
That’s also when the Powdery Mildew Risk Assessment Index (RAI), developed by University of California researchers, comes in handy. It relates the risk of disease development to air temperature, and simplifies and improves the task of assessing disease pressure and determining how often treatment is needed.
The higher the pressure, the shorter the interval between spays needed to control the disease. An index of 30 or less indicates low disease pressure, while an index of 30 to 50 shows intermediate pressure, and 60 or above reveals high disease pressure.
“Once the index rises to 50 or higher, it’s time to consider using a synthetic fungicide to control powdery mildew,” Dosier says. “The index removes the guesswork from trying to figure out the best timing for a chemical treatment.”
UC guidelines for controlling powdery mildew in grapes call for alternating treatment among products with different modes of action (MOA). No fungicides with identical MOAs should be applied more than twice in a row before rotating to one with a different MOA group number.
Dosier recommends including Orius 20 AQ, a sterol demethylation inhibitor (DMI), in rotation with fungicides with other modes of action, such as stobilurins and boscalin products, when treating grapes under high powdery mildew pressures.
Orius 20 AQ features a unique and novel solvent system that offers highly effective, economical control of powdery mildew and excellent crop safety, he says. Also, its label contains the less restrictive “Caution” signal word.
”This liquid, fast-acting product is absorbed through the leaves and fruit blossoms,” Dosier says.” It penetrates the cuticle for better control than dry formulations, and can be used to cure and prevent powdery mildew and other diseases.”
Over the past four decades, Jim Alfieri, a PCA with Wilbur-Ellis at Manteca, Calif., has developed a powdery mildew treatment program designed to help his grape growers in the northern San Joaquin Valley keep vines and fruit clean of powdery mildew from the start of shoot growth to grape harvest. It features the use of both sulfur and synthetic fungicides.
“Generally, if you go in early and start control of powdery mildew from the start, you won’t have problems later on,” he says.
Alfieri begins with two applications of wettable sulfur combined with copper. The first goes on when canes are about 4 to 6 inches long; the second follows 10 to 14 days later, when canes have reached about 12 inches in length. He includes a non-ionic spreader-sticker in the tank mix so the spray covers the leaves better and doesn’t wash off in the event of rain.
After that, he switches to sulfur dust, applying it in rotation with synthetic fungicides.
“These fungicides provide some systemic activity that will pick up any powdery mildew starting on the leaves,” Alfieri says.
By the time he starts fungicide applications, foliage growth tends to hinder penetration of the sprays into the vines. To improve coverage, he replaces the spreader-sticker with a silicone surfactant.
The fungicide treatments, which vary among products with different chemistries, provide about 14 to 21 days of protection. Then, the dust controls powdery mildew for about 7 to 10 days before he repeats the cycle. Alfieri continues this program until the grapes reach 12°Brix.
Getting good coverage of berries is critical to the success of fungicide applications, he says. It’s a matter of combining right nozzles and the right fan speed of the sprayer with the right tractor speed.
“You want the nozzles to create a spray particle size small enough so the fungicide gets into the nooks and crannies where the powdery mildew hangs out and prevents the disease from getting started,” Alfieri says.
“You have to use plenty of water and shoot the spray down from the top of the vines and up from the bottom. With the fans moving the leaves around, and traveling no faster than about 2½ miles per hour, you can get the material into the bunches. Most growers get too excited and try to cover too much ground too fast — as a result, their spray misses the target.”
To measure spray coverage, he attaches water-sensitive paper to the wires running through the canopy near berry. “If the paper has turned pretty much blue after the sprayer goes by, you’re getting good coverage,” he says.